In recent years, with increased demand for high-performance plastics, lots of polymers of various novel properties have been developed and come on the market. Liquid crystalline resins having characteristic parallel arrangement of molecular chains and showing optical anisotropy, such as liquid crystalline polyester, have received particular attention, because of their excellent moldability and mechanical properties, and their applications have been expanded to mechanical components and electric and electronic components. In particular, the liquid crystalline resins are favorably used for electric and electronic components requiring the high flowability, such as connectors.
With downsizing and refining the mechanical components and the electric and electronic components, molded products have ultra-thin wall thickness. Liquid crystalline polyester compositions filled with fibrous inorganic fillers have been used with an aim to improve the strength of the product and reduce the anisotropy.
Accompanied with reduction of the wall thickness of molded products, higher degrees of warpage reduction and dimensional accuracy have been demanded for materials. With the aim of improving anisotropy of molded products, the following liquid crystalline resin composition has been proposed (e.g., see JP H06-240115 A). This liquid crystalline resin composition is filled with 5 to 300 parts by weight of glass fiber having the average fiber diameter of 3 to 15 μm and the weight-average fiber length of 0.02 to 0.45 mm and satisfies the conditions that: (i) the ratio of specific glass fiber having the fiber length of greater than 1 mm is in the range of 0 to 8 percent by weight of the glass fiber; and (ii) the ratio of specific glass fiber having the fiber length of not greater than 0.1 mm is greater than 50 percent by weight of the glass fiber. The molded product obtained from this liquid crystalline resin composition, however, has the occurrence of filler lift that causes glass fiber to be lifted above the surface and accordingly has insufficient surface smoothness.
In the liquid crystalline resin composition, the high solidification rate of the liquid crystalline resin may cause long glass fibers dispersed in the composition to be present in the curved state. In the course of reflow of a molded product of this composition for surface mounting, the surface of the molded product is softened by heat, so that the curved glass fiber may be restored. This may cause the glass fibers near the surface to be lifted above the surface and thereby disadvantageously reduce the surface smoothness. Contact or slide between components during assembly of a molded product may cause peel-off or detachment of the filler of the liquid crystalline resin composition or the resin surface layer. Attachment of such peeled or detached pieces to the terminal may cause poor conduction, and attachment of such peeled or detached pieces to the lens may cause spots. The liquid crystalline polyester composition having the glass fiber length controlled to a specific range has accordingly been used. In many cases, pins are press-fit during assembly of connectors, so that the liquid crystalline resin composition has been required to have sufficient pin press-fit tolerance.
With the aim of preventing blister and reducing protrusion of glass fibers from the gate and from the fluid terminals, for example, one proposed liquid crystalline resin composition contains: (i) 5 to 50 percent by weight of glass fiber having the number-average fiber length of 50 to 120 μm and the content of the fiber length of 20 to 150 μm to be not lower than 80%; and (ii) 95 to 5 percent by weight of liquid crystalline polymer (e.g., see JP 2009-191088 A). With the aim of preventing filler detachment, one proposed resin molded product contains 5 to 250 parts by weight of fibrous filler having the fiber diameter of 5 to 15 μm and the number-average fiber length of 30 to 200 μm, relative to 100 parts by weight of liquid crystalline polymer (e.g., see JP 2008-239950 A). This proposed molded product, however, still has insufficient pin press-fit tolerance and insufficient warpage reduction.
To improve the weld strength and the surface smoothness of molded products, one proposed liquid crystalline resin composition contains 20 to 80 parts by weight of glass fiber having the maximum fiber length of not greater than 1000 μm and the weight-average fiber length of not less than 200 μm and not greater than 450 μm, relative to 100 parts by weight of liquid crystalline resin (e.g., see JP 2009-215530 A). This proposed resin composition, however, does not effectively prevent the filler lift and still gives the insufficient surface smoothness.
Recent size reduction and refinement of molded products requires satisfaction of the high degrees of surface smoothness, warpage reduction and pin press-fit tolerance. The conventional techniques, however, still does not sufficiently satisfy all these requirements. It could therefore be helpful to provide a liquid crystalline polyester composition that gives a molded product having high surface smoothness, high pin press-fit tolerance and less warpage.